The portable electronic devices (e.g., mobile phones), embedded with an image display panel (LCD panel, organic EL (Electro-Luminescence) panel, etc.), are generally provided with a cover glass for protecting the image display panel. For example, this type of cover glass for a potable electronic device is fabricated as follows. First, a large-sized plate glass is cut in a predetermined shape for fabricating a small-sized plate glass. Next, the small-sized plate glass is soaked in molten salt for chemical strengthening. Subsequently, a variety of functional layers (e.g., antireflective film) is formed on the surface of the chemically strengthened small-sized plate glass on an as-needed basis (see Japan Laid-open Patent Application Publication No. JP-A-2007-099557, especially paragraphs 0042, 0043, 0056 and 0057). In other words, a chemical strengthening step is executed after a cutting step in the art of the publication No. JP-A-2007-099557. Meanwhile, there has been proposed another art of executing the cutting step by means of etching in executing the cutting step and the chemical strengthening step in the same order as the art of the publication No. JP-A-2007-099557 (see International Patent Application No. WO2009/078406, especially claims).
In the art of the publication No. WO2009/078406, a plate glass is cut by means of wet etching (chemical etching). In addition, a cutting method using dry etching has been known (see Japan Laid-open Patent Application Publication No. JP-A-S63-248730, especially claims and right bottom section of page 3). In the art of the publication No. JP-A-S63-248730, it is also proposed that a variety of functional films are formed on a plate glass and the plate glass with functional films are subsequently cut by means of etching. However, scribe-cutting is generally more widely used than etching as a plate glass cutting method. It should be herein noted that the following drawbacks have been pointed out for executing scribe-cutting with respect to a tempered glass or a chemically strengthened glass (see Japan Laid-open Patent Application Publication No. JP-A-2004-83378, especially claim 1, paragraph 0007, and Description of Preferred Embodiments). The tempered glass is broken into pieces when being scribe-cut. On the other hand, the chemically strengthened glass cannot be divided along scribe-lines. Also, glass substrates, obtained by scribe-cutting of the chemically strengthened glass, can be broken with a load smaller than an estimated withstand load. To appropriately cut a chemically strengthened glass along scribe-lines, the art of the publication No. JP-A-2004-83378 proposes a chemically strengthened glass with the following configuration. The chemically strengthened glass includes a compressive stress layer with a thickness in a range of 10 to 30 μm for withstanding against a compressive stress in a range of 30 to 60 kgf/mm2.
Except for the aforementioned arts, the following art has been known as an exemplary conventional art of the present invention (see Japan Laid-open Patent Application Publication No. JP-A-2008-247732, especially claims). In the art, a large-sized chemically strengthened glass is cut by means of physical cutting such as laser cutting or scribe-cleaving.
A glass is cut by means of a scribing or etching treatment, and an ion-exchanging treatment is executed in manufacturing a chemically strengthened plate rectangular glass product (i.e., glass substrate) with at least a functional layer disposed thereon on an as-needed basis as exemplified in the publication No. JP-A-2007-99557, the application No. WO2009/078406 and so forth. Therefore, the glass substrate is ion-exchanged from the front-face side, the back-face side and the edge sides. Specifically, a compressive stress layer is formed on each corner part of the glass substrate by means of the ion-exchanging treatment from three directions (i.e., the front-face/back-face side and two edge sides). Therefore, the compressive stress layer on each corner part of the glass substrate has a thickness much greater than that of the compressive stress layer on the other part of the glass substrate. Accordingly, strain (stress) tends to be concentrated on the compressive stress layer on each corner part of the glass substrate. In other words, each corner part is inferior to the other part from the perspective of impact resistance.
In view of the above, the present invention has been produced. It is an object of the present invention to provide a glass substrate, including corner parts with good impact resistance, of a cover glass for a portable electronic device, an image display unit for a portable electronic device using the glass substrate, a portable electronic device using the substrate, and a method of manufacturing a glass substrate of a cover glass for a portable electronic device.